No. 474] DRAGONFLIES IN BRACKISH WATER 



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a few hours. Those hatched in the 1.010 solution had apparently 

 gained no further power of resistance but succumbed as quickly 

 as those from fresh water. This test was repeated after two weeks 

 but with the same result, they still were overcome as readily as 

 when first hatched. 



The above experiments indicate that there is in the Odonata 

 a very definite barrier to their assumption of marine life, and that 

 this barrier remains unchanged during the life of the individual. 

 That it is the same for all species has not yet been determined, and 

 it may be that forms such as Micrathijria berenice which are limited 

 in distribution to the coastline have a higher limit than those 

 species which occur in the interior only. As to the nature of the 

 barrier we are entirely in the dark. It may be that the eggs and 

 nymphs of Odonata are able to prevent the osmosis of salt in 

 solution up to a certain point, but it seems more probable that the 

 metabolism is interfered with only by salt in solution above a 

 certain density. Whether other groups of insects are similarly 

 restricted is also unknown. 

 Columbia University 



